Cobalt Ruthenium Liner Deposition is a advanced interconnect metallization layer employing conformal atomic layer deposition of cobalt or ruthenium to create diffusion barriers and improve void-free metal fill in high-aspect-ratio features — enabling next-generation interconnect scaling.
Barrier Layer Function and Requirements
Interconnect metal (copper) diffuses rapidly into surrounding dielectric and silicon at elevated temperature through grain boundaries and surfaces; diffusion creates leakage paths, threshold voltage shifts, and device degradation. Barrier layers (typical thickness 5-20 nm) prevent copper diffusion: barrier material must exhibit: (1) negligible copper solubility, (2) slow diffusion coefficient for copper, and (3) adequate adhesion to both copper and dielectric. Traditional Ta/TaN barriers exhibit excellent diffusion resistance but higher resistivity (100+ μΩ-cm for TaN) contributing significant series resistance in scaled features. Cobalt and ruthenium alternatives offer lower resistivity (8-10 μΪ-cm bulk values) reducing interconnect RC delay penalty.
Cobalt Liner via ALD
- ALD Chemistry: Atomic layer deposition of cobalt employs cyclic exposure to cobalt precursor (dicobalt octacarbonyl, Co₂(CO)₈, or cobalt cyclopentadienyl, CoCp) and reducing agent (H₂ plasma or borane)
- Monolayer Control: Sequential precursor/reducing agent pulses deposit ~0.1-0.3 Ångströms per cycle; depositing 5-50 nm liners requires 200-500 cycles, each cycle 0.5-2 seconds
- Conformal Coverage: ALD produces uniform thickness on high-aspect-ratio features through diffusion-limited growth; enables conformal liners on 10:1 aspect ratio vias
- Adhesion: Cobalt strong adhesion to SiO₂ (high interfacial energy) and copper (forms Co-Cu alloy at interface); superior to traditional tantalum
Ruthenium Seed Layer Approach
Ruthenium alternative approach: thin ruthenium (5-20 nm) deposited via ALD or CVD serves dual function: diffusion barrier for copper (low copper solubility in Ru, slow copper diffusion rate) and nucleation seed for subsequent copper electrochemical plating (ECP). Ruthenium provides superior conductivity (~7 μΩ-cm) versus tantalum nitride (~100 μΩ-cm), reducing resistance contribution. Thickness optimization critical: thinner ruthenium reduces resistance but diminishes diffusion barrier effectiveness; typical designs employ 10 nm balancing both requirements.
Process Integration and Bottom-Up Fill
- Via Structure: High-aspect-ratio vias (depth/diameter >3:1) present filling challenges: conventional copper ECP deposits from bottom-up, prone to void formation if current distribution non-uniform
- Cobalt-Enhanced Fill: Cobalt liner ALD coating conformal on all surfaces including via sidewalls and bottom; improves copper nucleation uniformity across bottom and sidewalls
- Current Distribution: Uniform cobalt underlayer improves cathodic current distribution during copper ECP; reduced current density variations minimize void risk
- Fill Quality: Optimized cobalt liner thickness (10-20 nm) with subsequent copper ECP achieves void-free fill for 5-10:1 aspect features; thicker cobalt (>30 nm) begins resistance penalties
Resistance Contribution and Scaling Impact
Total interconnect resistance includes: metal bulk (copper), contact/barrier interface, and barrier/liner material. For 48 nm pitch wires (typical 7 nm technology node), 100 nm deep interconnect: barrier/liner contribution ~10-20% of total resistance if optimized. Traditional TaN liner 20 nm thick contributes ~50-100 mΩ per line; cobalt/ruthenium liner equivalent thickness reduces contribution to ~10-30 mΩ. Cumulative savings across millions of interconnects significant for circuit delay and power. Process window tight — exceeding ~50 nm liner thickness begins eroding overall resistance advantage.
Thermal Stability and Reliability
- Copper-Cobalt Interaction: Cobalt demonstrates superior thermal stability versus traditional barriers; cobalt-copper mutual diffusion minimal up to ~400°C
- Interface Reactions: Cobalt-SiO₂ interface exhibits weak thermal oxidation; copper-cobalt interface remains stable with minimal interdiffusion
- Electromigration Performance: Cobalt barriers enhance copper electromigration resistance through improved interface stability; expected lifetime improvements 2-3x versus tantalum barriers
Alternative Liner Materials and Advanced Concepts
Emerging research: tungsten-based liners (W, W-Ru alloys) providing superior diffusion resistance at cost of increased resistivity; tradeoff calculus between improved reliability versus speed penalty. Graphene-based barriers (emerging concept) demonstrate exceptional copper blocking in early research, but manufacturing feasibility unproven. Self-assembled monolayer (SAM) barriers approaching theoretical limit of single-atom resistance contribution, but practical copper integration challenging.
Closing Summary
Cobalt and ruthenium liners represent a critical advancement enabling scaled interconnect geometry through conformal diffusion barriers with controlled resistivity, maintaining copper's superior conductivity while preventing destructive diffusion — essential for sub-20 nm pitch interconnect hierarchies supporting terahertz clock performance targets.